ACF uses an existing field in repeaters to do this, but repeaters have the same fields on every row so you can select which one to show. Flex fields can have different fields on every row do there is not simple way to just let us select a field to be shown. Basically we need to do the work of deciding what field will be shown, even if it will be the same field every time because it’s just an admin field to let the user enter something to identify it so they know what it is when collapsed. I just had to do this recently on a complex site.
